Target control in extreme ultraviolet lithography systems using aberration of reflection image

ABSTRACT

A method of controlling an extreme ultraviolet (EUV) lithography system is disclosed. The method includes irradiating a target droplet with EUV radiation, detecting EUV radiation reflected by the target droplet, determining aberration of the detected EUV radiation, determining a Zernike polynomial corresponding to the aberration, and performing a corrective action to reduce a shift in Zernike coefficients of the Zernike polynomial.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of and claims priorityunder 35 U.S.C. § 120 to U.S. non-provisional application Ser. No.16/926,489 filed Jul. 10, 2020, the entire contents of which areincorporated herein by reference.

BACKGROUND

The wavelength of radiation used for lithography in semiconductormanufacturing has decreased from ultraviolet to deep ultraviolet (DUV)and, more recently to extreme ultraviolet (EUV). Further decreases incomponent size require further improvements in resolution of lithographywhich are achievable using extreme ultraviolet lithography (EUVL). EUVLemploys radiation having a wavelength of about 1-100 nm.

One method for producing EUV radiation is laser-produced plasma (LPP).In an LPP-based EUV source, a high-power laser beam is focused on smalldroplet targets of metal, such as tin, to form a highly ionized plasmathat emits EUV radiation with a peak maximum emission at 13.5 nm. Theintensity of the EUV radiation produced by LPP depends on theeffectiveness with which the high-powered laser can produce the plasmafrom the droplet targets. Precise synchronization of the pulses of thehigh-powered laser with generation and movement of the droplet targetsis desired to improve the efficiency of an LPP-based EUV radiationsource.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a schematic view of an EUV lithography system with a laserproduced plasma (LPP)-based EUV radiation source, in accordance withsome embodiments of the present disclosure.

FIGS. 2A, 2B, 2C, 2D, 2E, and 2F schematically illustrate the movementof target droplet by the pre-pulse in X-Z and X-Y planes respectively.

FIG. 3 schematically illustrates the laser guide optics and the focusingapparatus used in the EUV lithography system illustrated in FIG. 1, inaccordance with an embodiment.

FIG. 4 illustrates the first 15 Zernike polynomials ordered verticallyby radial degree and horizontally by azimuthal frequency.

FIG. 5 shows an exemplary schematic view of the EUV lithography systemillustrated in FIG. 1.

FIG. 6 is a graph indicating variation in the coefficients of the 4^(th)Zernike polynomial as determined by measurements using the return beamdiagnostic, according to embodiments of the disclosure.

FIG. 7 illustrates signal flow in the EUV lithography system illustratedin FIG. 5, according to embodiments of the disclosure.

FIG. 8 illustrates a droplet illumination module including a radiationsource, a tilt control mechanism, and a slit control mechanism.

FIGS. 9A and 9B show a EUV data analyzing apparatus according to anembodiment of the present disclosure.

FIG. 10 illustrates a flow-chart of a method of controlling an extremeultraviolet (EUV) lithography system, in accordance with an embodimentof the present disclosure.

FIG. 11 illustrates a flow-chart of a method of controlling an extremeultraviolet (EUV) lithography system, in accordance with an embodimentof the present disclosure.

FIG. 12 illustrates a flow-chart of a method of controlling an extremeultraviolet (EUV) lithography system, in accordance with an embodimentof the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“being made of” may mean either “comprising” or “consisting of.” In thepresent disclosure, a phrase “one of A, B and C” means “A, B and/or C”(A, B, C, A and B, A and C, B and C, or A, B and C), and does not meanone element from A, one element from B and one element from C, unlessotherwise described.

The present disclosure is generally related to extreme ultraviolet (EUV)lithography systems and methods. More particularly, it is related toapparatuses and methods for improved target control to obtainedincreased EUV energy by controlling an excitation laser used in a laserproduced plasma (LPP)-based EUV radiation source. The excitation laserheats metal (e.g., tin) target droplets in the LPP chamber to ionize thedroplets to a plasma which emits the EUV radiation. For increased EUVenergy, a majority of the excitation laser has to be incident on thetarget droplets to improve EUV output and conversion efficiency. Thus,the shape of the excitation laser, the angle of incidence of theexcitation laser, and the profile of the laser beam has to be consideredin order to obtain increased EUV energy.

Existing methods consider the relative position between the targetdroplets and the excitation laser without considering the shape of theexcitation laser, change in the angle of incidence of the excitationlaser (also referred to as pointing error), and the profile of the laserbeam. Thus, EUV energy drop due to these issues are not detected. Sincethese issues cannot be detected, the excitation laser control systemand/or the droplet generator cannot be controlled to address theseissues and thereby compensate for the reduced EUV energy.

Embodiments of the present disclosure are directed to controlling therelative position (e.g., direction of travel) of the excitation laserand the position of the target droplet based on the angle of incidenceof the laser beam on the target droplet and the profile of the laserbeam.

FIG. 1 is a schematic view of an EUV lithography system with a laserproduced plasma (LPP)-based EUV radiation source, in accordance withsome embodiments of the present disclosure. The EUV lithography systemincludes an EUV radiation source 100 to generate EUV radiation, anexposure tool 200, such as a scanner, and an excitation laser source300. As shown in FIG. 1, in some embodiments, the EUV radiation source100 and the exposure tool 200 are installed on a main floor MF of aclean room, while the excitation laser source 300 is installed in a basefloor BF located under the main floor. Each of the EUV radiation source100 and the exposure tool 200 are placed over pedestal plates PP1 andPP2 via dampers DMP1 and DMP2, respectively. The EUV radiation source100 and the exposure tool 200 are coupled to each other by a couplingmechanism, which may include a focusing unit.

The lithography system is an extreme ultraviolet (EUV) lithographysystem designed to expose a resist layer by EUV light (alsointerchangeably referred to herein as EUV radiation). The resist layeris a material sensitive to the EUV light. The EUV lithography systememploys the EUV radiation source 100 to generate EUV light, such as EUVlight having a wavelength ranging between about 1 nm and about 100 nm.In an example, the EUV radiation source 100 generates an EUV light witha wavelength centered at about 13.5 nm. In the present embodiment, theEUV radiation source 100 utilizes a mechanism of laser-produced plasma(LPP) to generate the EUV radiation.

The exposure tool 200 includes various reflective optical components,such as convex/concave/flat mirrors, a mask holding mechanism includinga mask stage, and wafer holding mechanism. The EUV radiation generatedby the EUV radiation source 100 is guided by the reflective opticalcomponents onto a mask secured on the mask stage. In some embodiments,the mask stage includes an electrostatic chuck (e-chuck) to secure themask. Because gas molecules absorb EUV light, the lithography system forthe EUV lithography patterning is maintained in a vacuum or a-lowpressure environment to avoid EUV intensity loss.

In the present disclosure, the terms mask, photomask, and reticle areused interchangeably. In the present embodiment, the mask is areflective mask. In an embodiment, the mask includes a substrate with asuitable material, such as a low thermal expansion material or fusedquartz. In various examples, the material includes TiO₂ doped SiO₂, orother suitable materials with low thermal expansion. The mask includesmultiple reflective layers (ML) deposited on the substrate. The MLincludes a plurality of film pairs, such as molybdenum-silicon (Mo/Si)film pairs (e.g., a layer of molybdenum above or below a layer ofsilicon in each film pair). Alternatively, the ML may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configurable to highly reflect the EUV light. The mask mayfurther include a capping layer, such as ruthenium (Ru), disposed on theML for protection. The mask further includes an absorption layer, suchas a tantalum boron nitride (TaBN) layer, deposited over the ML. Theabsorption layer is patterned to define a layer of an integrated circuit(IC). Alternatively, another reflective layer may be deposited over theML and is patterned to define a layer of an integrated circuit, therebyforming an EUV phase shift mask.

The exposure tool 200 includes a projection optics module for imagingthe pattern of the mask on to a semiconductor substrate with a resistcoated thereon secured on a substrate stage of the exposure tool 200.The projection optics module generally includes reflective optics. TheEUV radiation (EUV light) directed from the mask, carrying the image ofthe pattern defined on the mask, is collected by the projection opticsmodule, thereby forming an image on the resist.

In various embodiments of the present disclosure, the semiconductorsubstrate is a semiconductor wafer, such as a silicon wafer or othertype of wafer to be patterned. The semiconductor substrate is coatedwith a resist layer sensitive to the EUV light in presently disclosedembodiments. Various components including those described above areintegrated together and are operable to perform lithography exposingprocesses.

The lithography system may further include other modules or beintegrated with (or be coupled with) other modules.

As shown in FIG. 1, the EUV radiation source 100 includes a targetdroplet generator 115 and a laser produced plasma (LPP) collector 110,enclosed by a chamber 105. The target droplet generator 115 generates aplurality of target droplets DP, which are supplied into the chamber 105through a nozzle 117. In some embodiments, the target droplets DP aretin (Sn), lithium (Li), or an alloy of Sn and Li. In some embodiments,the target droplets DP each have a diameter in a range from about 10microns (μm) to about 100 μm. For example, in an embodiment, the targetdroplets DP are tin droplets, each having a diameter of about 10 μm,about 25 μm, about 50 μm, or any diameter between these values. In someembodiments, the target droplets DP are supplied through the nozzle 117at a rate in a range from about 50 droplets per second (i.e., anejection-frequency of about 50 Hz) to about 50,000 droplets per second(i.e., an ejection-frequency of about 50 kHz). For example, in anembodiment, target droplets DP are supplied at an ejection-frequency ofabout 50 Hz, about 100 Hz, about 500 Hz, about 1 kHz, about 10 kHz,about 25 kHz, about 50 kHz, or any ejection-frequency between thesefrequencies. The target droplets DP are ejected through the nozzle 117and into a zone of excitation ZE at a speed in a range from about 10meters per second (m/s) to about 100 m/s in various embodiments. Forexample, in an embodiment, the target droplets DP have a speed of about10 m/s, about 25 m/s, about 50 m/s, about 75 m/s, about 100 m/s, or atany speed between these speeds. The remnants (residue) after theinteraction of the target droplets DP with the excitation laser LR2 arecollected in a tin catcher TC located below the target droplet generator115.

The excitation laser LR2 generated by the excitation laser source 300 isa pulse laser. The laser pulses LR2 are generated by the excitationlaser source 300. The excitation laser source 300 includes a lasergenerator 310, laser guide optics 320, and a focusing apparatus 330. Insome embodiments, the laser source 310 includes a carbon dioxide (CO₂)or a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source witha wavelength in the infrared region of the electromagnetic spectrum. Forexample, the laser source 310 has a wavelength of 9.4 μm or 10.6 μm, inan embodiment. The laser light LR1 generated by the laser generator 310is guided by the laser guide optics 320 and focused into the excitationlaser LR2 by the focusing apparatus 330, and then introduced into theEUV radiation source 100.

In some embodiments, the excitation laser LR2 includes a pre-heat laserand a main laser. In such embodiments, the pre-heat laser pulse(interchangeably referred to herein as the “pre-pulse”) is used to heat(or pre-heat) a given target droplet to create a low-density targetplume with multiple smaller droplets, which is subsequently heated (orreheated) by a pulse from the main laser, generating increased emissionof EUV light.

In various embodiments, the pre-heat laser pulses have a spot size about100 μm or less, and the main laser pulses have a spot size in a range ofabout 150 μm to about 300 μm. In some embodiments, the pre-heat laserand the main laser pulses have a pulse-duration in the range from about10 ns to about 50 ns, and a pulse-frequency in the range from about 1kHz to about 100 kHz. In various embodiments, the pre-heat laser and themain laser have an average power in the range from about 1 kilowatt (kW)to about 50 kW. The pulse-frequency of the excitation laser LR2 ismatched with the ejection-frequency of the target droplets DP in anembodiment.

The laser light LR2 is directed through windows (or lenses) into thezone of excitation ZE. The windows adopt a suitable materialsubstantially transparent to the laser beams. The generation of thelaser pulses is synchronized with the ejection of the target droplets DPthrough the nozzle 117. As the target droplets move through theexcitation zone, the pre-pulses heat the target droplets and transformthem into low-density target plumes. A delay between the pre-pulse andthe main pulse is controlled to allow the target plume to form and toexpand to an optimal size and geometry. In various embodiments, thepre-pulse and the main pulse have the same pulse-duration and peakpower. When the main pulse heats the target plume, a high-temperatureplasma is generated. The plasma emits EUV radiation EUV, which iscollected by the collector mirror 110. The collector 110 furtherreflects and focuses the EUV radiation for the lithography exposingprocesses performed through the exposure tool 200.

FIG. 2A schematically illustrates the movement of target droplet DPrelative to the collector 110 after being irradiated by the pre-pulsePP. A target droplet DP is sequentially irradiated by the pre-pulse PPand the main pulse MP. When the target droplet DP travels along X-axisin a direction “A” from the droplet generator DG to the zone ofexcitation ZE, the pre-pulse PP exposing the target droplet DP causesthe target droplet DP to change its shape into, for example, a pancakeand introduce a Z-axis component to its direction of travel in the X-Zplane.

The laser-produced plasma (LPP) generated by irradiating the targetdroplet DP with the laser beams PP, MP presents certain timing andcontrol problems. The laser beams PP, MP must be timed so as tointersect the target droplet DP when it passes through the targetedpoint. The laser beams PP, MP must be focused on each of their focuspositions, respectively, where the target droplet DP will pass. Theposition of the zone of excitation ZE and parameters such as, forexample, laser power, time delay between the main pulse and thepre-pulse, focal point of the pre-pulse and/or main pulse, may bedetermined when an EUV radiation source 100 is set up. The actualposition of the zone of excitation ZE and the aforementioned parametersare then adjusted during wafer exposure using a feedback mechanism invarious embodiments. However, these parameters can change over time dueto various factors such as, for example, separation between the mainpulse MP and the pre-pulse PP, shape of the excitation laser, theprofile of the laser beam, mechanical and/or electrical drift in theradiation source, instability of the droplet generator, changes inchamber environment.

FIG. 2B illustrates an exemplary optical metrology for misalignment inthe x-axis OMX. OMX is defined by a distance in the x-axis between adroplet and the focal point of the pre-pulse PP. Similarly, FIG. 2Cillustrates an exemplary optical metrology for misalignment in they-axis OMY. OMY is defined by a distance in the y-axis between thedroplet and the focal point of the pre-pulse PP. FIG. 2D furtherillustrates an exemplary optical metrology for misalignment in thez-axis OMZ. Similar to OMX and OMY, OMZ is defined by a distance in thez-axis between a droplet and the focal point of the pre-pulse PP. FIG.2E illustrates an exemplary optical metrology for misalignment in radiusOMR. The x-axis is in the direction of motion by the droplet from thedroplet generator 115. The z-axis is along the optical axis A1 (FIG. 1)of the collector mirror 110. The y-axis is perpendicular to the x-axisand the z-axis.

As shown in FIG. 2F, the target droplet DP is ejected from a dropletgenerator 115 travelling in a direction to a tin catcher TC. When suchmechanical and/or electrical drift occurs in the radiation source, thepre-pulse laser PP causes the target droplet DP to expand in a directionwith an angle with respect to a direction of incidence from thepre-pulse laser beam. This gives a rise to a target droplet DP2 whichhas expanded to form a football-like shape shown in FIG. 2E. In such anembodiment, a spatial separation between the pre-pulse PP and themain-pulse MP, MPPP separation, is defined as a distance between thefocal point of the pre-pulse PP and the focal point of the main-pulseMP, which is a 3-D vector contributed by x, y, and z sections. Forexample, as shown in FIG. 2F, MPPPx is a distance along the x-axis ofthe MPPP separation and MPPPz is a distance along the z-axis of the MPPPseparation.

FIG. 3 schematically illustrates the laser guide optics 320 and thefocusing apparatus 330 used in the EUV lithography system illustrated inFIG. 1, in accordance with an embodiment. As illustrated, the laserguide optics 320 includes a forward beam diagnostic (FBD) 302, a returnbeam diagnostic (RBD) 304, and a plurality of mirrors M301, M303, M300,and M330. The forward beam diagnostic 302 and the return beam diagnostic304 include a device such as wavefront sensor for measuring theaberrations of an optical wavefront. Some non-limiting types ofwavefront sensors include a Shack-Hartmann wavefront sensor, aphase-shifting Schlieren technique, a wavefront curvature sensor, apyramid wavefront sensor, a common-path interferometer, a Foucaultknife-edge test, a multilateral shearing interferometer, a Ronchitester, and a shearing interferometer.

The forward beam diagnostic 302 and the return beam diagnostic 304, andmirrors M301, M303 constitute a final focus metrology (FFM) module 350.The signal from the final focus metrology (FFM) module 350 is used as acontrol signal and may be connected with an actuator to control one ofthe mirrors of the focusing apparatus 330, such as for example, themirror M150 in the optical path before the laser hits the targetdroplets DP. In some embodiments, the mirror M150 is the last mirrorbefore the laser hits the target droplets DP. The mirror M150 is asteerable mirror and is adjustable in 3 axis.

The mirrors M301, M303, M300, and M330 are arranged (or otherwiseconfigured to) guide incident laser light in a desired direction. Thefocusing apparatus 330 includes a plurality of mirrors M310, M320, M130, M140, and M150. The mirrors M310, M320, M130, M140, and M150 arearranged (or otherwise configured to) guide incident laser light in adesired direction. The focusing apparatus 330 also includes windows W10,W20, and W30. The windows W10 and W20, and mirrors M310 and M320 are inan environment that is under atmospheric pressure conditions. Themirrors M130, M140, and M150 are in vacuum. The window W10 is located atan entry point into the focusing apparatus 330 and receives laser lightfrom the laser guide optics 320. The window W20 is located between theatmospheric pressure environment and vacuum.

The forward beam diagnostic 302 receives the laser light LR1 generatedby the laser generator 310. The forward beam diagnostic 302 analyzes thelaser light LR1 generated by the laser generator 310. Some of the laserlight LR1 is guided from the window W10 of the focusing apparatus 330 onto the mirror M300. From the mirror M300, the laser light LR1 isincident on the mirror M301. The laser light LR1 is guided by the mirrorM301 to the forward beam diagnostic 302. Thus, the forward beamdiagnostic 302 receives the laser light LR1 before the laser light LR1interacts with the target droplets DP. The forward beam diagnostic 302analyzes the wavefront of the laser light LR1.

As illustrated, the laser light LR1 passes through the window W10 andwindow W20 of the focusing apparatus 330 and is incident on the mirrorM130. The mirror M130 is arranged (or otherwise configured) such thatthe laser light LR1 is reflected on to the mirror M140. The mirror M140is arranged (or otherwise configured) such that the laser light LR1received from the mirror M130 is reflected on to the mirror M150. Themirror M150 is arranged (or otherwise configured) such that the laserlight LR1 obtained from the mirror M140 is reflected on to the targetdroplets DP. The laser light thus guided by the mirrors M130, M140, andM150 is focused into the excitation laser LR2 by the focusing apparatus330, and then introduced into the EUV radiation source 100 (FIG. 1).

After interacting with the target droplets DP, the excitation laser LR2is dispersed and a return beam of excitation laser LR2 is guided back towindow W10 via the mirrors M150, M140, and M130 and window W20. From thewindow W10, the return beam of excitation laser LR2 travels to mirrorM310 via window W30. In some embodiments, the window W10 is a diamondwindow. The mirror M310 guides the return beam to mirror M303 viamirrors M320 and M330. The return light is guided to the return beamdiagnostic 304 using mirror M303. The return beam diagnostic 304receives the return beam and analyzes the return beam, morespecifically, the optical wavefront of the return beam.

In analyzing the return beam of excitation laser LR2, the return beamdiagnostic 304 generates a plurality of Zernike polynomials. EachZernike polynomial describes specific form of surface deviation that canbe fit to specific forms of wavefront deviations (aberrations). Byincluding a plurality of Zernike polynomials (commonly referred to asterms), wavefront deformation can be described to a desired degree ofaccuracy.

FIG. 4 illustrates the first 15 Zernike polynomials ordered verticallyby radial degree and horizontally by azimuthal frequency. Table 1 belowlists the different Zernike polynomials and the aberration type obtainedfrom each polynomial.

TABLE 1 Zemike Polynomial Index Aberration Type Z₀ ⁰ 1 Piston Z₁ ¹ 2 Tip(X-Tilt, horizontal tilt) Z₁ ⁻¹ 3 Tilt (Y-Tilt, vertical tilt) Z₂ ⁰ 4Defocus Z₂ ⁻² 5 Oblique Astigmatism Z₂ ² 6 Vertical Astigmatism Z₃ ⁻¹ 7Vertical Coma Z₃ ¹ 8 Horizontal Coma Z₃ ⁻³ 9 Vertical Trefoil Z₃ ³ 10Horizontal Trefoil Z₄ ⁰ 11 Primary Spherical Z₄ ² 12 Vertical secondaryastigmatism Z₄ ⁻² 13 Oblique secondary astigmatism Z₄ ⁴ 14 Verticalquadrafoil Z₄ ⁻⁴ 15 Oblique quadrafoil

The return beam diagnostic 304 is configured to measure aberration ofthe received wavefront (radiation) of the return beam of excitationlaser LR2 and quantify the aberration using the Zernike polynomials. Thereturn beam diagnostic 304 analyzes the received wavefront to determinethe Zernike coefficients of the Zernike polynomial that best fits thespecific wavefront deviation. Based on the shift in the Zernikecoefficients, the change in the beam profile can be determined. FIG. 5shows an exemplary schematic view of the EUV lithography systemillustrated in FIG. 1. As illustrated, a control signal 501corresponding to the change in the beam profile is generated by thefinal focus metrology (FFM) module 350. As discussed above, the controlsignal 501 is connected with an actuator 505 to control one of themirrors of the focusing apparatus 330, such as for example, the mirrorM150 in the optical path before the laser hits the target droplets DP.Thus, by using the control signal 501, the targeting control isoptimized to maximize generation of EUV energy.

In some embodiments, the feedback mechanism, illustrated in FIG. 5, mayfurther send a notification based on the change in the beam profile. Insome embodiments, the notification includes a spatial separation betweenthe pre-pulse and the main-pulse. In some embodiments, the notificationalso includes a time delay between the pre-pulse and the main-pulse. Insome embodiments, the notification also includes an angle of a steerablemirror coupled to the radiation source. In some embodiments, based onthe generating the notification, the feedback further sends thenotification to a first external device associated with a steerablemirror controller and a second external device associated with a timedelay controller.

FIG. 6 is a graph 600 indicating variation in the Zernike coefficientsof the 4^(th) Zernike polynomial as determined by measurements using thereturn beam diagnostic 304, according to embodiments of the disclosure.In the embodiment illustrated in FIG. 6, the return beam diagnostic 304analyzes the wavefront of the return beam of excitation laser LR2 anddetermines that the excitation laser LR2 incident on the target dropletsDP has an aberration of the type defocus, which corresponds to the4^(th) Zernike polynomial. The return beam diagnostic 304 determines thevariation (line 602) in the Zernike coefficients 611 of the 4^(th)Zernike polynomial. The variation in the Zernike coefficients 611 withina desired range is considered acceptable and no corrective action isperformed. However, for a variation beyond the desired range, asindicated by the dip 605 in the line 602, one or more corrective actionsare undertaken. A corrective action includes actuating the actuator 505to change a position of the mirror M150 based on the control signal 501.Alternatively or additionally, one or more of the OMX, OMY and OMZdistances can be adjusted to minimize the change in the beam profile andthereby improve the interaction between the excitation laser LR2 and thetarget droplets DP.

Also illustrated are the images 606 and 608 obtained by the return beamdiagnostic 304 before and after the dip 605, respectively. Asillustrated, the defocus aberration determined in image 606 is reducedin image 608 due to the corrective actions. The example in FIG. 6illustrates how the 4^(th) Zernike polynomial can be used to detect thedefocus aberrations in the optical wavefront, according to embodiments.However, embodiments are not limited thereto. Other Zernike polynomialscan be used to detect corresponding aberrations, and one or morecorrective actions can be taken to mitigate the detected aberrations.Thus, by improving the interaction between the target droplets and theexcitation laser LR2, the conversion efficiency can be maximized, andvariations (fluctuations) in the EUV energy can be minimized.

FIG. 7 illustrates signal flow in the EUV lithography system illustratedin FIG. 5, according to embodiments of the disclosure. The desiredposition of the zone of excitation ZE and parameters such as, forexample, laser power, time delay between the main pulse and thepre-pulse, focal point of the pre-pulse and/or main pulse, may bedetermined when an EUV radiation source 100 is set up, and therebydefine the set point 702 of the EUV lithography system.

A control signal 701 corresponding to one or more of the parameters isprovided to one or more components of the EUV lithography system tocontrol the position of the zone of excitation ZE. For instance, one ormore of the position of the droplet generator 115 and trajectory of theexcitation laser LR2 are adjusted to maximize the interaction betweenthe target droplets DP and the excitation laser LR2 to maximize thegeneration of EUV energy. In some embodiments, the control signal 701 isthe control signal 501 provided to the actuator 505 to control one ofthe mirrors of the focusing apparatus 330, such as for example, themirror M150 in the optical path before the excitation laser LR2 hits thetarget droplets DP.

However, these parameters can change over time due to various factorssuch as, for example, separation between the main pulse MP and thepre-pulse PP, shape of the excitation laser, the profile of the laserbeam, mechanical and/or electrical drift in the radiation source,instability of the droplet generator, changes in chamber environment.

The interaction between the excitation laser LR2 and the target dropletsDP is analyzed at 704. The target droplets DP reflect and/or scatter thelight (excitation laser LR2, in this case) incident upon it. Thereflected and/or scattered light is detected for example, at a dropletdetection module 420 (FIG. 1). In some embodiments, the dropletdetection module 420 includes a photodiode designed to detect lighthaving a wavelength of the light from a droplet illumination module 410(FIG. 1). In various embodiments, the droplet illumination module 410 isa continuous wave laser or a pulsed laser having emitting light of adesired wavelength.

It is determined whether an intensity of the detected light (i.e., lightreflected and/or scattered by the target droplet) is within anacceptable range. In some embodiments, the determination is based on avalue of current and/or voltage produced by photodiode of the dropletdetection module 420 when it receives the light reflected and/orscattered by the target droplet DP. In some embodiments, the dropletdetection module 420 includes a logic circuit programmed to generate aprescribed signal 706 when the detected intensity is not within anacceptable range. For example, the prescribed signal 706 is generatedwhen the detected intensity is less than a certain threshold value. Theprescribed signal 706 is indicative of the relative position of theexcitation laser LR2 and the target droplets DP.

If the intensity of the detected light is not within the acceptablerange, a parameter of the droplet illumination module 410 is adjusted(e.g., automatically) to increase or decrease the intensity of lightirradiating the target droplet so as to ultimately bring the intensityof the detected light within the acceptable range.

In various embodiments, the parameter of the droplet illumination module410 includes, for example, an input voltage and/or current to the lightsource (e.g., laser) in the droplet illumination module 410, a width ofa slit controlling the amount of light exiting the droplet illuminationmodule 410, an aperture of the droplet illumination module 410, and avalue of angle and/or tilt of the droplet illumination module 410. Insome embodiments, the parameter is adjusted using a controller that isprogrammed to control various parameters of the droplet illuminationmodule 410. For example, in an embodiment, the controller is coupled toa slit controlling the amount of light exiting the droplet illuminationmodule 410 and/or a mechanism that controls the tilt/angle of thedroplet illumination module 410. In such embodiments, the controller iscoupled to the droplet detection module 420 and adjusts the width of theslit and/or the tilt of the droplet illumination module 410 in responseto the prescribed signal 706 generated by the droplet detection module420 when the intensity of the detected light is not within theacceptable range. In other embodiments, the controller is coupled to theactuator 505 and provides control signal 701 to the actuator 505 tocontrol one of the mirrors of the focusing apparatus 330, such as forexample, the mirror M150 in the optical path before the laser hits thetarget droplets DP.

In some embodiments, the controller is a logic circuit programmed toreceive the prescribed signal 706 from the droplet detection module 420,and depending on the prescribed signal 706 transmit control signals toone or more components (e.g., the slit and/or tilt control mechanismdescribed elsewhere herein) of the droplet illumination module 410 toautomatically adjust one or more parameters of the droplet illuminationmodule 410 and/or to adjust one of the mirrors of the focusing apparatus330.

Referring briefly to FIG. 8, illustrated is the droplet illuminationmodule 410 including a radiation source 415, a tilt control mechanism413 and a slit control mechanism 417. The tilt control mechanism 413(also referred to herein as “auto tilt”) controls the tilt of theradiation source 415. In various embodiments, the auto tilt 413 is astepper motor coupled to the radiation source 415 (e.g., laser) of thedroplet illumination module 410 and moves the radiation source 415 tochange the angle of incidence at which light (or radiation) L isincident on the target droplet DP (and in effect changing the amount oflight R reflected and/or scattered by the target droplet DP into thedroplet detection module 420). In some embodiments, the auto tilt 413includes a piezoelectric actuator.

The slit control mechanism 417 (also referred to herein as “auto slit”)controls the amount of light exiting the radiation source 415. In anembodiment a slit or an aperture 414 is disposed between the radiationsource 415 and the zone of excitation ZE at which the target droplet DPis irradiated. When, for example, the controller 450 determines that theintensity of light detected at droplet detection module 420 is lowerthan the acceptable range, the controller 450 moves the slit controlmechanism 417 such that a wider slit is provided in the path of lightexiting the radiation source 415, allowing more light to irradiate thetarget droplet DP and increasing the detected intensity. On the otherhand, if it is determined that the intensity of light detected at thedroplet detection module 420 is higher than the acceptable range, thecontroller 450 moves the slit control mechanism 417 such that a narrowerslit is provided in the path of light exiting the radiation source 415,thereby reducing the detected intensity. In such embodiments, parameterof the droplet illumination module 410 adjusted by the controller 450 isthe width of the aperture 414 in the path of light L irradiating thetarget droplet DP.

Returning to FIG. 7, generation of the prescribed signal 706 thuschanges the set point 702 of the EUV lithography system. The controlsignal 701 is correspondingly changed based on the change is the setpoint 702. The change in the control signal 701 thus actuates the slitcontrol mechanism, tilt control mechanism, and/or the actuator 505 tocausing corresponding changes in the amount of light exiting theradiation source 415 and the angle of incidence at which light isincident on the target droplet DP.

In addition to the above techniques to maximize generation of EUVenergy, embodiments of the disclosure are also directed to utilizing theZernike shift in the wavefront of the light reflected after interactionbetween the excitation laser LR2 and the target droplets DP. Morespecifically, embodiments measure aberration of the received wavefront(radiation) and quantify the aberration using the Zernike polynomials inorder to obtain the Zernike shift in the reflected wavefront.Accordingly, at 704, the return beam diagnostic 304 determines theZernike shift in the reflected wavefront and a corresponding controlsignal 708 is generated. In some embodiments, the control signal 708 isthe control signal 501 (illustrated in FIG. 5) that corresponds to thechange in the beam profile and is generated by the final focus metrology(FFM) module 350. As discussed above, the control signal 501 isconnected with the actuator 505 to control one of the mirrors of thefocusing apparatus 330, such as for example, the mirror M150 in theoptical path before the laser hits the target droplets DP. Thus, thetargeting control is optimized to maximize generation of EUV energy.

FIGS. 9A and 9B show a EUV data analyzing apparatus according to anembodiment of the present disclosure. FIG. 9A is a schematic view of acomputer system that controls an operation of the final focus metrology(FFM) module 350 and the return beam diagnostic 304 for detecting one ormore aberrations in the return image and performing one or morecorrective actions described above. The foregoing embodiments may berealized using computer hardware and computer programs executed thereon.In FIG. 9A, a computer system 900 is provided with a computer 901including an optical disk read only memory (e.g., CD-ROM or DVD-ROM)drive 905 and a magnetic disk drive 906, a keyboard 902, a mouse 903,and a monitor 904.

FIG. 9B is a diagram showing an internal configuration of the computersystem 900. In FIG. 9B, the computer 901 is provided with, in additionto the optical disk drive 905 and the magnetic disk drive 906, one ormore processors 911, such as a micro processing unit (MPU), a ROM 912 inwhich a program such as a boot up program is stored, a random accessmemory (RAM) 913 that is connected to the MPU 911 and in which a commandof an application program is temporarily stored and a temporary storagearea is provided, a hard disk 914 in which an application program, asystem program, and data are stored, and a bus 915 that connects the MPU911, the ROM 912, and the like. Note that the computer 901 may include anetwork card (not shown) for providing a connection to a LAN.

The program for causing the computer system 900 to execute the functionsof the EUV data analyzing apparatus in the foregoing embodiments may bestored in an optical disk 921 or a magnetic disk 922, which are insertedinto the optical disk drive 905 or the magnetic disk drive 906, and betransmitted to the hard disk 914. Alternatively, the program may betransmitted via a network (not shown) to the computer 901 and stored inthe hard disk 914. At the time of execution, the program is loaded intothe RAM 913. The program may be loaded from the optical disk 921 or themagnetic disk 922, or directly from a network.

In the programs, the functions realized by the programs do not includefunctions that can be realized only by hardware in some embodiments. Forexample, functions that can be realized only by hardware, such as anetwork interface, in an acquiring unit that acquires information or anoutput unit that outputs information are not included in the functionsrealized by the above-described programs. Furthermore, a computer thatexecutes the programs may be a single computer or may be multiplecomputers.

Embodiments of the disclosure provide numerous advantages over theexisting systems and methods. Zernike terms which correlate with theshift in the X, Y, and/or Z direction of the excitation beam and/or thetarget droplet direction, and the beam profile change are used toquantify the change in shape of beam and the aberration. By using thisas a feedback, an additional feedback loop is generated to compensatethe shifting error from the return image instead of only consideringrelative targeting position. The angle to incidence of the excitationlaser is detected and minimized so that a more stable EUV energy can beobtained.

It will be understood that not all advantages have been necessarilydiscussed herein, no particular advantage is required for allembodiments or examples, and other embodiments or examples may offerdifferent advantages.

An embodiment of the present disclosure is a method 1000 of operating anextreme ultraviolet (EUV) lithography system according to the flowchartillustrated in FIG. 10. It is understood that additional operations canbe provided before, during, and after processes discussed in FIG. 10,and some of the operations described below can be replaced oreliminated, for additional embodiments of the method. The order of theoperations/processes may be interchangeable and at least some of theoperations/processes may be performed in a different sequence. At leasttwo or more operations/processes may be performed overlapping in time,or almost simultaneously.

The method includes an operation S1010 of irradiating a target dropletwith laser radiation. In operation S1020, laser radiation reflected bythe target droplet is detected. In operation S1030, aberration of thedetected laser radiation is determined. In operation S1040, a Zernikepolynomial corresponding to the aberration is determined. In operationS1050, a corrective action is performed to reduce a shift in at leastone of Zernike coefficients of the Zernike polynomial.

Another embodiment of the present disclosure is a method 1100 ofoperating an extreme ultraviolet (EUV) lithography system according tothe flowchart illustrated in FIG. 11. It is understood that additionaloperations can be provided before, during, and after processes discussedin FIG. 11, and some of the operations described below can be replacedor eliminated, for additional embodiments of the method. The order ofthe operations/processes may be interchangeable and at least some of theoperations/processes may be performed in a different sequence. At leasttwo or more operations/processes may be performed overlapping in time,or almost simultaneously.

The method includes an operation S1110 of detecting excitation radiationreflected by a target droplet generated by a droplet generator of theEUV lithography system. The EUV lithography system also includes an EUVradiation source for generating EUV radiation that includes anexcitation radiation source. The excitation radiation from theexcitation radiation source interacts with the target droplet. Inoperation S1120, aberration of the detected excitation radiation isdetermined. In operation S1130, a plurality of Zernike polynomials aregenerated. In operation S1140, one or more Zernike polynomials from theplurality of Zernike polynomials that correspond to the aberration aredetermined. In operation S1150, a corrective action is performed toreduce a shift in at least one of Zernike coefficients of the one ormore Zernike polynomials.

Another embodiment of the present disclosure is a method 1200 ofoperating an extreme ultraviolet (EUV) lithography system according tothe flowchart illustrated in FIG. 12. It is understood that additionaloperations can be provided before, during, and after processes discussedin FIG. 12, and some of the operations described below can be replacedor eliminated, for additional embodiments of the method. The order ofthe operations/processes may be interchangeable and at least some of theoperations/processes may be performed in a different sequence. At leasttwo or more operations/processes may be performed overlapping in time,or almost simultaneously.

The method includes an operation S1210 of detecting excitation radiationreflected by a target droplet generated by a droplet generator of theEUV lithography system. In operation S1220, aberration of the detectedexcitation radiation are determined. In operation S1230, a plurality ofZernike polynomials are generated. In operation S1240, one or moreZernike polynomials from the plurality of Zernike polynomials thatcorrespond to the aberration are determined. In operation S1250, a shiftin at least one of Zernike coefficients of the one or more Zernikepolynomials is determined. In operation S1260, a change in a beamprofile of the excitation radiation based on the shift in the Zernikecoefficients is detected.

According to one aspect of the present disclosure, a method ofcontrolling an extreme ultraviolet (EUV) lithography system includesirradiating a target droplet with laser radiation and detecting laserradiation reflected by the target droplet. The method also includesdetermining aberration of the detected laser radiation, determining aZernike polynomial corresponding to the aberration, and performing acorrective action to reduce a shift in at least one of Zernikecoefficients of the Zernike polynomial. In one or more otherembodiments, the corrective action includes generating a control signalto actuate one or more components of the EUV lithography system toadjust an interaction between the laser radiation and the targetdroplet. In one or more other embodiments, the interaction between thelaser radiation and the target droplet is adjusted by changing aposition of a droplet generator of the EUV lithography system, changinga trajectory of the laser radiation, or both. In one or more otherembodiments, the one or more component includes an actuator andadjusting the interaction between the laser radiation and the targetdroplet includes controlling a focal point of laser radiation using theactuator. In one or more other embodiments, the actuator is connected toa steerable mirror, and the corrective action includes adjusting thesteerable mirror using the actuator to adjust an interaction between thelaser radiation and the target droplet. In one or more otherembodiments, the corrective action includes adjusting an angle ofincidence of the laser radiation. In one or more other embodiments, themethod further includes generating a plurality of Zernike polynomials,and selecting the Zernike polynomial from the plurality of Zernikepolynomials. The selected Zernike polynomial correspond to theaberration. In one or more other embodiments, the method furtherincludes detecting a change in a beam profile of the EUV radiation basedon the shift in the Zernike coefficients. In one or more otherembodiments, the method further includes generating a control signalcorresponding to the change in the beam profile, controlling an actuatorof the EUV lithography system using the control signal, and adjusting asteerable mirror of the EUV lithography system using the actuator tochange an optical path of the laser radiation. In one or more otherembodiments, the shift in the Zernike coefficients is reduced such thatEUV energy generated by an interaction of the laser radiation and targetdroplet is increased. In one or more other embodiments, the laserradiation includes a CO₂ laser.

According to yet another aspect of the present disclosure, an apparatusfor extreme ultraviolet (EUV) lithography includes a droplet generatorconfigured to generate target droplets, and an EUV radiation source forgenerating EUV radiation including an excitation radiation source. Theexcitation radiation from the excitation radiation source interacts withthe target droplets. The apparatus also includes a final focus modulethat is configured to detect excitation radiation reflected by thetarget droplet, determine aberration of the detected excitationradiation, generate a plurality of Zernike polynomials, determine one ormore Zernike polynomials from the plurality of Zernike polynomials thatcorrespond to the aberration, and perform a corrective action to reducea shift in at least one of Zernike coefficients of the one or moreZernike polynomials. In one or more other embodiments, the apparatusfurther includes a steerable mirror. The steerable mirror is a lastmirror in an optical path of the excitation radiation before theexcitation radiation interacts with the target droplet. The apparatusalso includes an actuator configured to control the steerable mirror.The final focus module is further configured to adjust the steerablemirror using the actuator to adjust an interaction between theexcitation radiation and the target droplet. In one or more otherembodiments, the steerable mirror is adjustable in 3 axis. In one ormore other embodiments, the final focus module is further configured todetect a change in a beam profile of the excitation radiation based onthe shift in the Zernike coefficients. In one or more other embodiments,the final focus module is further configured to reduce the shift in theZernike coefficients such that EUV energy generated by an interaction ofthe excitation radiation and target droplet is increased.

According to another aspect of the present disclosure, a non-transitory,computer-readable medium includes computer readable instructions storedin a memory which, when executed by a processor of a computer direct thecomputer to control a final focus module of an extreme ultraviolet (EUV)lithography apparatus to perform a method. The method includes detectingexcitation radiation reflected by a target droplet, determiningaberration of the detected excitation radiation, generating a pluralityof Zernike polynomials, determining one or more Zernike polynomials fromthe plurality of Zernike polynomials that correspond to the aberration,determining a shift in at least one of Zernike coefficients of the oneor more Zernike polynomials, and detecting a change in a beam profile ofthe excitation radiation based on the shift in the Zernike coefficients.In one or more other embodiments, the method further includes generatinga control signal corresponding to the change in the beam profile,controlling an actuator of the EUV lithography system using the controlsignal, adjusting a steerable mirror of the EUV lithography system usingthe actuator to change an optical path of the excitation radiation. Inone or more other embodiments, the steerable mirror is a last mirror inthe optical path before the excitation radiation hits the targetdroplet. In one or more other embodiments, the steerable mirror isadjustable in 3 axis.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method of controlling an extreme ultraviolet(EUV) lithography system, the method comprising: irradiating a targetdroplet with laser radiation; guiding a return beam of laser radiationto metrology device of the EUV lithography system; determiningaberration of the return beam using the metrology device; determiningZernike coefficients of a Zernike polynomial that corresponds to theaberration; and determining a change in a beam profile of the returnbeam based on at least one of Zernike coefficients of the Zernikepolynomial.
 2. The method of claim 1, wherein the change in the beamprofile is determined based on a shift in the at least one Zernikecoefficient.
 3. The method of claim 2, wherein the shift in the at leastone Zernike coefficient is reduced such that EUV energy generated by aninteraction of the laser radiation and target droplet is increased. 4.The method of claim 1, further comprising: performing a correctiveaction by reducing a shift in the at least one Zernike coefficient. 5.The method of claim 4, wherein the corrective action includes adjustingan angle of incidence of the laser radiation.
 6. The method of claim 4,wherein the corrective action includes generating a control signal toactuate one or more components of the EUV lithography system to adjustan interaction between the laser radiation and the target droplet. 7.The method of claim 6, wherein the interaction between the laserradiation and the target droplet is adjusted by changing a position of adroplet generator of the EUV lithography system, changing a trajectoryof the laser radiation, or both.
 8. The method of claim 6, wherein theone or more component includes an actuator and adjusting the interactionbetween the laser radiation and the target droplet includes controllinga focal point of laser radiation using the actuator.
 9. The method ofclaim 8, wherein the actuator is connected to a steerable mirror, andthe corrective action includes adjusting the steerable mirror using theactuator to adjust the interaction between the laser radiation and thetarget droplet.
 10. The method of claim 1, further comprising:generating a plurality of Zernike polynomials; and selecting the Zernikepolynomial from the plurality of Zernike polynomials, the selectedZernike polynomial corresponding to the aberration.
 11. The method ofclaim 1, further comprising: generating a control signal correspondingto the change in the beam profile; controlling an actuator of the EUVlithography system using the control signal; and adjusting a steerablemirror of the EUV lithography system using the actuator to change anoptical path of the laser radiation.
 12. The method of claim 1, whereinthe laser radiation includes a CO₂ laser.
 13. An apparatus for extremeultraviolet (EUV) lithography, comprising: a droplet generatorconfigured to generate target droplets; an EUV radiation source forgenerating EUV radiation including an excitation radiation source,excitation radiation from the excitation radiation source interactingwith the target droplets; a system of mirrors for guiding the EUVradiation after interaction with the target droplets; a final focusmodule that receives the EUV radiation after interaction with the targetdroplets from the system of mirrors and is configured to: determineaberration of the EUV radiation that is received; generate a pluralityof Zernike polynomials; determine one or more Zernike polynomials fromthe plurality of Zernike polynomials that correspond to the aberration;determine a shift in at least one of Zernike coefficients of the one ormore Zernike polynomials; and reduce the shift by controlling one ormore mirrors in the system of mirrors to change a trajectory of theexcitation radiation prior to interacting with the target droplets. 14.The apparatus of claim 13, wherein the one or more mirrors in the systemof mirrors includes a steerable mirror, the steerable mirror being alast mirror in an optical path of the excitation radiation before theexcitation radiation interacts with the target droplets; and the finalfocus module controls the steerable mirror to adjust an interactionbetween the excitation radiation and the target droplets.
 15. Theapparatus of claim 14, wherein the steerable mirror is adjustable in 3axis.
 16. The apparatus of claim 13, wherein the final focus module isfurther configured to detect a change in a beam profile of theexcitation radiation based on the shift in the Zernike coefficients. 17.The apparatus of claim 13, wherein the final focus module is furtherconfigured to reduce the shift in the Zernike coefficients such that EUVenergy generated by an interaction of the excitation radiation andtarget droplets is increased.
 18. A non-transitory, computer-readablemedium comprising computer readable instructions stored in a memorywhich, when executed by a processor of a computer direct the computer tocontrol a final focus module of an extreme ultraviolet (EUV) lithographyapparatus to perform a method, the method comprising: detectingexcitation radiation reflected by a target droplet; determiningaberration of the detected excitation radiation; generating a pluralityof Zernike polynomials; determining one or more Zernike polynomials fromthe plurality of Zernike polynomials that correspond to the aberration;determining a shift in at least one of Zernike coefficients of the oneor more Zernike polynomials; detecting a change in a beam profile of theexcitation radiation based on the shift in the Zernike coefficients; andgenerating a control signal to adjust a trajectory of an excitationradiation prior to interacting with the target droplet based on thechange in the beam profile.
 19. The non-transitory, computer-readablemedium of claim 18, wherein the control signal controls an actuator forchanging a position of one or more mirrors that guide the excitationradiation prior to the excitation radiation interacting with the target.20. The non-transitory, computer-readable medium of claim 19, whereinthe one or more mirrors includes a steerable mirror that is a lastmirror in an optical path of the excitation radiation, the steerablemirror being adjustable in 3 axis.